Enter the Electric Sail

by Paul Gilster on May 8, 2013

Some years back at the Aosta interstellar conference I had the pleasure of being on a bus making its way at night through the Italian alps with Pekka Janhunen sitting immediately in front of me. Janhunen (Finnish Meteorological Institute) is the developer of the electric sail concept soon to be tested by the ESTCube-1 satellite, which launched last night aboard a Vega rocket from the Kourou spaceport in French Guiana. Our group had been talking about interstellar issues all day at the conference and now, headed back to the hotel following a memorable dinner at high elevation, I was curious whether an electric sail had interstellar applications.

The immediate answer seemed to be no, given that the highest velocities Janhunen had been talking about for the idea were about 100 kilometers per second, much faster than Voyager 1’s 17 kilometers per second, but a long way short of what we would like to see on an interstellar flight. But the ever thoughtful Pekka pointed out to me that as a means of deceleration, electric sails might have a future, braking against the stellar wind from a destination star. Deceleration being a huge problem for any interstellar probe, the idea has stuck with me ever since.

Image: The electric sail is a space propulsion concept that uses the momentum of the solar wind to produce thrust. Credit: Alexandre Szames.

What an electric sail would do is to ride the stream of charged particles flowing out from the Sun, and fast missions to the outer system could thus be implemented if we get the system into full gear. The ESTCube-1 satellite, the work of Estonian students testing out Janhunen’s ideas, uses a long wire that maintains a steady electric potential as its means of interacting with the solar wind. Janhunen, in an article in IEEE Spectrum, calls ESTCube-1 “…the first attempted experiment to measure the Coulomb drag experienced by a charged wire or tether in moving plasma.”

ESTCube-1’s tether is a 50 micrometer wide, 10 meter long wire made out of four strands of aluminum that will gradually be deployed from the satellite in a process that could take as much as a week. Once deployed, the tether will be charged and variations in the satellite’s rotation rate will, if all goes well, reveal the interactions between it and atmospheric ions. But future electric sails will soon be deploying longer wires. A follow-up to ESTCube-1 called Aalto-1 is designed around a 100-meter tether. This one is also a student project, built at Aalto University in Finland and designed in part to test charged tethers as a means of deorbiting small satellites.

Assuming the concept passes its initial muster, we can look forward to upsized missions using tethers up to 20 kilometers long, deploying as many as a hundred of these from a single spacecraft. This is the design that, in computer simulations, yields potential speeds of 100 kilometers per second, fast enough to get a payload into the nearby interstellar medium in about fifteen years. With a spacecraft like this, keeping the sail’s wires in a 20 kV positive potential allows the sail to ride the solar wind ions while making issues of deployment relatively simple.

A sail like this is surprisingly efficient. From a page on the concept maintained by Pekka Janhunen:

The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun. This is about 5000 times weaker than the solar radiation pressure. Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft’s electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind.

As with any sail, the effect is small but cumulative and yields serious velocities over time:

…by equipping a 1000 kg spacecraft with 100 such wires, one may produce acceleration of about 1 mm/s2. After acting for one year, this acceleration would produce a significant final speed of 30 km/s. Smaller payloads could be moved quite fast in space using the electric sail, a Pluto flyby could occur in less than five years, for example. Alternatively, one might choose to move medium size payloads at ordinary 5-10 km/s speed, but with lowered propulsion costs because the mass that has to launched from Earth is small in the electric sail.

ESTCube-1 will help us measure the forces exerted on its single tether by the ionospheric ram flow acting on the satellite, a flow that substitutes for the solar wind in the case of this small CubeSat mission. The Aalto-1 test will occur next year if all goes well, and we will then have to see how the electric sail stands up compared to its solar sail competition. More on electric sail concepts tomorrow, when I want to look at important questions of stability.

Similar to the electric sail, has there been anymore development of the M2P2 (Mini-magnetosphere plasma propulsion) by Robert Winglee ? It had proposed speeds of 180KM/sec and no need for kilometer long wires. The added bonus I see is the mini-magnetosphere would also protect astronauts from different forms of radiation hazards.

ESTCube-1 team issued a press release regarding the progress and future plans. The satellite works fine, fully charged and acts according to calculation. The test of the e-Sail will start in June . On their press release they say so:

“What will happen next? At the end of May, the satellite will start transmitting photos of Earth. Testing of the solar sail is planned to start in July. For the freshest news, follow ESTCube-1 on Facebook.”

What I surprisingly discovered that we tend to have peculiar affinity towards the space. Can’t say why but it is present in our pop music in many ways. Beside the ones I’ve already shared here the ESTCubers reminded of these songs on Estonian music arena. Iiris should be well known in nearby region. Tenfold Rabbit is a new band on the scene.

There is one particular chapter captured imagination:
“6.2 Possible missions
Assuming that no technological obstacles will arise that markedly alter the performance of the electric sail from the above estimates, which type of missions could benefit from it? The electric sail much resembles the solar sail in that it provides small but inexhaustible thrust which is directed outward from the Sun, with a modest control of the thrust direction allowed (probably by a few tens of degrees). First and foremost the electric sail can thus be used for missions going outward in the solar system and aiming for >50 km/s final speed, such as missions going out of the heliosphere and fast and cheap flyby missions of any target in the outer solar system. Secondly, by inclining the sail to some angle it can also be used to spiral inward in the solar system to study e.g. Mercury and Sun. Also a nonzero inclination with respect to the ecliptic plane is possible to achieve which may be beneficial
for observing the Sun. Also the return trip back to Earth from the inner solar system is possible, as is cruising back and forth in the inner solar system and visiting multiple targets such as asteroids. Thirdly, the electric sail could be used to implement a solar wind monitoring spacecraft which
is placed permanently between Earth and Sun at somewhere else than the Lagrange point, thus providing a space weather service with more than one hour of warning time. Propulsion and data taking phases probably must be interleaved because ion measurements are not possible when the platform
is charged to high positive voltage, although the plasma density and dynamic pressure of the solar wind can probably be sensed by an electron detector and accelerometer even when the electric sail voltage is turned on.
Once accelerated to a high outward speed an electric sailing spacecraft cannot by itself stop to orbit a remote target because the radial component of the thrust is always positive. For stopping under those circumstances one has to use aerocapture or some other traditional technique. Although
the electric sail does not provide a marked speed benefit for such missions, being propellantless it might still provide cost saving; this remains to be studied. In interstellar space the plasma flow is rather slow. Thus the electric sail cannot be used for acceleration, but it can instead be used for braking the spacecraft. There are some concepts such as the laser or microwave sail which are designed to “shoot” a small probe at ultrahigh speed towards e.g. a remote solar system. In these concepts the power source is at Earth so that the accelerated probe needs no propulsive
energy source. Stopping the probe at the remote target is very difficult, however, if one has to rely on power beamed from the starting point. The electric sail might then provide a feasible stopping mechanism for such mission concepts. In other words, one would shoot a probe to another solar system at ultrahigh speed using a massive and powerful laser or microwave source installed in near-Earth space, brake the probe before the target by the electric sail action in the interstellar plasma and finally explore the extrasolar planetary system with the help of the electric sail and the stellar wind. A similar idea was proposed by Zubrin and Andrews (1990)
for their magnetic version of the solar wind sail.”

Thethers certainly have a lot of potential for many things. It is very nice to see electric sail technology take flight, because the plasma physics flying the solar wind certainly needs experiments.

One of my dreams is to hold a competition for the fastest human object, a “Bacca Nigra” or “Black Pearl” competition of space flight to produce the fastest vehicle we can build with existing technology. Obviously solar sail (sundiver type) and electrical sail technology (including the use of some of the materials we pursue for SE tethers) are high up in the potential propulsion options, but there may be competitive systems. There could be a handicap factor for the money invested in the vehicle as well as a jackpot that would be originally put together via crowd funding on something like Kickstarter, but could grow over time.

What is the possibility of increasing the proton density to propel teh vehicle, much like the use of lasers for interstellar sails?

I’m thinking of some sort of collimation of the solar proton flux using electric fields, or the creation of an artificial, higher density, proton beam. Obviously both techniques are working against the tendency of the protons to repel each other, but perhaps there is some scope for enhancement?

Greg, the M2P2 system performance envelope violates the MHD assumptions needed for Winglee et al’s computations to be valid. Three different papers – including one by Pekka Janhunen – pointed out the flaws in about 2005. The Japanese are still researching magnetoplasma sails but they don’t perform as spectacularly as Winglee had hoped.

Those nearly 4 orders of magnitude of lesser pressure for solar wind vs. solar light really diminish the potential of this technology. I believe a solar light sail has been calculated to achieve much higher escape velocities than those mentioned here for the solar wind sail, or am I mistaken?

Some recent talk here about the Fermi paradox has had me wondering if we currently have the technology level to send a probe containing long lived earth bacteria to impact on a rocky planet in the HZ orbiting a nearby star. Such a probe would need to decelerate at the target system and be able to navigate its self onto a collision course with the planet.

I’ve been stuck on thinking what propulsion mechanism could be used, that would still operate after the many thousands of years in transit. However it sounds like an Electric Sail could do it!

A 1000kg probe would have more than enough payload space to contain bacteria and at least some rudimentary shielding for re-entry. Its also small enough to be economically launched by our current generation of launchers – even by a private organisation.

100km/s will get you to Alpha Centauri B in 13,169 years. Now, that’s a hell of a long time. But if we stick to purely electronic systems we should be able to engineer something that can still operate over those time frames, if all it needs to do is crash land.

So, it seems that the ability to send earth based life to other star systems will very soon be in our grasp … if we want to.

I wonder if the problem that Dmitri mentioned above (an electric sail being unable to stop at a planet) might be solvable by using a “convertible” electric/magnetic sail, at least in the cases of planets like Jupiter, Saturn, Uranus, and Neptune that possess strong global magnetic fields? It might work like this:

A wagon wheel-like sail (a circular wire “hoop” with numerous wire “spokes” running inward from the hoop to the spacecraft at the hub) would be electrically charged as a whole, in order to function as an electric sail. For operation as a magnetic sail, switches at appropriate wire junctions would be opened or closed, making the wire hoop (and some of the spokes) a closed circuit through which current would flow, thus creating a magnetic field around the (now magnetic) sail. Also:

The spacecraft could travel from Earth out to, say, Jupiter using the electric sail mode. Approaching Jupiter, it would switch to the magnetic sail mode to brake into orbit around Jupiter, pushing against one pole of the planet’s magnetic field. The spacecraft’s Earth departure trajectory could be designed so that it would be angled slightly above or below the plane of the Earth’s orbit, which would enable the spacecraft to approach Jupiter from above its North magnetic pole (or from below its South magnetic pole) in order to maximize the magnetic repulsion braking effect. Once in circum-Jovian orbit, the spacecraft could use its sail in magnetic sail mode to “push and pull” on Jupiter’s magnetic poles in order to change its orbit. It might also be able to use its sail in electric sail mode, using the Io torus and Io-Jupiter flux tube plasma particles for propulsion.

One technical aspect I’ve not seen mention of anywhere yet, is the reactive force acting on the long wire (as the equal and opposite force to the one creating thrust). Unless the wire is stiff enough, won’t it bend the sail against the wind (just as an Earthly sailboat’s sails stretch and flex against the wind)? If the wires are thin and flexible enough to easily be wound out, I’m guessing they’ll be pushed by the wind more into a cone around the spaceship, which reduces their effectiveness (less surface area presented perpendicular to the wind’s direction) when sailing away from the wind; but helps when sailing at non-perpendicular angles. There’s the general fluid-dynamics physics at work here, I’m not particularly knowledgeable on but those who design sails/masts for racing yachts will be.
Hope this is taken account of in their calculations!

Potential ways to mitigate this: use materials that gain rigidity with electric current/field, can be the same electricity that gives the sail its propulsive power.

Those nearly 4 orders of magnitude of lesser pressure for solar wind vs. solar light really diminish the potential of this technology. I believe a solar light sail has been calculated to achieve much higher escape velocities than those mentioned here for the solar wind sail, or am I mistaken?

I’ve seen escape velocities in the range of 500 kilometers per second for a close-pass ‘Sundiver’ sail, and you’re right that the electric sail can’t match that. Janhunen thinks its real potential is within the Solar System, and interstellar applications would be limited to deceleration at the target.

“Some recent talk here about the Fermi paradox has had me wondering if we currently have the technology level to send a probe containing long lived earth bacteria to impact on a rocky planet in the HZ orbiting a nearby star. Such a probe would need to decelerate at the target system and be able to navigate its self onto a collision course with the planet.

[snip]

“So, it seems that the ability to send earth based life to other star systems will very soon be in our grasp … if we want to.”

Should we if we can? How do we know the target system will not only support Earth life but what if it already has some of its own? How would we feel if an ETI dropped some life capsules on Earth and let their cargo loose?

I wish these questions would get asked more often in parallel with these technical thought experiments.

James Jason Wentworth: Wires this thin and long cannot possibly be stiff. The preferred way to keep the sail unfolded is by rotation. Centrifugal force will straighten the wires out into the wind. Because the wind is so incredibly weak, not much force is needed at all to achieve this.

As for magnetic breaking, these wires are too thin and have much too high a resistance to be of much use in generating magnetic fields. However, if charged up to a very high voltage, they may produce enough Lorentz force in the magnetic field around Jupiter to capture and keep a probe circling at much higher than orbital velocity. In such a forced orbit, velocity could gently be shed by the electric sail effect until normal orbit is achieved.

You could also envision going the other way, using the solar wind (or some other means of external propulsion available around Jupiter) to slowly accelerate, all the while staying close to Jupiter in a forced Lorentz orbit. Some calculations I once did indicate that nearly relativistic velocities might be achievable this way. After reaching maximum velocity in forced orbit, the charge is turned off and the craft shoots out towards the stars. It would require really thin and really strong wires, such as can only be made by carbon fiber. Lots of them, too. Most of the mass of the craft needs to be wires.

Using the solar wind for propulsion you can, of course, not reach velocities higher than that of the wind, which I think is around 400 km/s. An alternative means of propulsion would be a relativistic electron beam, driven by solar or nuclear power, which would do double duty: charge maintenance and propulsion. The speed limit for such a drive would be c, and the rocket equation would not apply: Eectrons constitute free reaction mass, they are picked up by the sail as it discharges to the interplanetary plasma.

A light sail could also be used, but it seems awkward to have to carry both a light and wind sail, plus an electron gun is still needed anyway for keeping the wind sail charged.

I agree with ljk that we should find out about life in other star systems before we go sending our own there. Moral aspects aside (although they shouldn’t be) there are important scientific questions that can only be answered by searching for and (if found) studying life on many worlds.

Mart Noorma, the head of the student satellite programme, has put it nicely:

“ESTCube-1 is 10 centimetres wide and has a 10-metre-long wire just half the width of a human hair. It is within the Earth’s magnetosphere, so is shielded from the solar wind, but it will still interact with charged particles, says Mart Noorma of Tartu University in Estonia, who helped develop the satellite.

Once the wire is fully extended and powered up, the satellite’s rotation rate should alter, letting the team measure the thrust generated by the electric sail. If the tests are successful, the hope is that a full-sized craft with 100 wires, each 20 kilometres long, could reach speeds of 30 kilometres per second, fast enough to get to Pluto in under five years.

Smaller sails could act as a brake for retired satellites, slowing them down enough to fall safely back to Earth.”

There is an ESA broadcast Mart Noorma making introduction w/ animation of ESTCube-1 on backgrond orbiting and showing the principles of its operations – http://youtu.be/0Dke2dLj4JA?t=1h1m56s

The objective is both to test e-Sail concept and feasibility. The Earth’s EM in this case is not an obstacle as Sun’s particles get through and they can evaluate how the e-Sail performs. The same sale can be used as a plasma breaker. I don’t know how it works and asked for materials on which I was referred to the same http://electric-sailing.fi/ Pekka Janhunen has foreseen the e-Sail design so it acts as propulsion away from the Sun and as a plasma breaker approaching the destination’s atmosphere. Of course all the intricate details can be better outlined by the team members. The staellite will stay active for a year and the e-Sail test commence in June (or maybe July).

When I listen the broadcast must say it’s starting to sink-in it’s actually a miracle that in 2005/2006 someone came to the idea doing it and we’ve got on good terms w/ ESA. 100 students participated. Built from scratch.

For those who are interested in ESTCube-1 hardware & technical aspects, there is a paper on it – versita.metapress.com/content/q6v67278p817763q/

Also all following information, relevant papers and related information will be updated on their homepage – http://www.estcube.eu

Electrons constitute free reaction mass, they are picked up by the sail as it discharges to the interplanetary plasma.

You are suggesting using the wires as an electric electron scoop and accelerating them for propulsion? Is this easier than protons? If the electric sail captured both electrons and protons in the solar wind (opposite charges on the alternate wires), it could accelerate protons and neutralize their charge like an ion engine. The question is whether the scoop makes more sense than just using stored propellant.

You are suggesting using the wires as an electric electron scoop and accelerating them for propulsion? Is this easier than protons? If the electric sail captured both electrons and protons in the solar wind (opposite charges on the alternate wires), it could accelerate protons and neutralize their charge like an ion engine. The question is whether the scoop makes more sense than just using stored propellant.

Electrons are much easier than protons. Firstly, in order to use the solar wind, we need a positive charge on the wire, so only electrons will be attracted to it. Second, only electrons can flow easily through the wire and cables to reach the electron gun. Protons would have to be captured and transported, without adding any weight to the already very flimsy wires.

Of course, in general, ions are much preferred for propulsion, but given that 1) only electrons are freely availble, 2) An electron gun is already needed for any craft based on charged wires, and c) electrons can be accelerated much faster than ions, I suspect electron propulsion is worth a second look, at least in this special situation.

Alternating charges are neither feasible nor useful. All the momentum of the solar wind is in the positive particles, so a sail with enough constant positive charge to turn them around 180 degrees is the best you can do. All the energy needed by this approach goes into re-ejecting electrons that hit the wire, to maintain the charge. The power required depends on the plasma density, not on the amount of acceleration. There is a correlation though, because more plasma usually means more wind and more acceleration. Thin wires are critical: thinner wire mean both less power (less electrons absorbed) and more acceleration (less mass), for a double whammy advantage. Of course this is limited be the wire strength, an ultrathin wire is not good if it breaks. For this reason, carbon fibers (such as CNT) are the material of choice.

“THE FIRST PHOTO TAKEN BY ESTCUBE-1 IN SPACE has been released today by the ESTCube-1 team! The hard work of the first two weeks has paid off and the CAM team, leaded by the University of Tartu Computer Technology graduate student Henri Kuuste has this to say: The camera works perfectly and so do all the other subsystems, needed for taking the photo. The first image was captured on May 15 over the Mediterranean Sea, showing the sea, Sahara desert, and Tunisia. And from behalf of the ESTCube-1 team, we thank all our friends in Facebook for your interest and constant support – this space photo is dedicated to you!”

The camera is self assembled industrial grade VGA resolution for monitoring the e-Sail w/ additional objective take pictures of Earth.

“Trivia: the camera was mostly developed as a bachelor thesis of Computer Technology (Arvutitehnika) at the University of Tartu (Henri Kuuste, ESTCube-1 Tether End Mass Imaging System Design and Assembly), which you can read here. There was a question about the camera resolution: it is standard VGA, 640 by 480 pixels. We could have easily installed higher resolution camera, but we just would not be able to download the images in reasonable time down to the Earth. Read all the details about the camera from Henri’s thesis. . Additional comment to the last one: if remote sensing of the Earth would be the main mission of ESTCube-1, we could have a different camera and also implemented a faster data downlink, at S-band for example. But it is not – we want to demonstrate a compact and efficient imaging system for proximity operations in space, which could be used on the other spacecraft in the future as well.”

A bit of reality – today Ecuadorian Space Agency (EXA) lost connection w/ its cube satellite Pegaso after NORAD gave them Cosmic Debris Collision Warning. It’s same as ESTCube-1 – 10cm x 10cm x 10cm, 1.2 kg. A thing such small gets hit by space debris from the Soviet rocket is sad. The satellite remains on the orbit and EXA people hope to restore the communication.

ESTCube-1 team complains only about over active Sun’s CME which may fry the cubesat electronics. Fingers crossed it won’t happened. From other perspective the prospects for successful e-Sail experiment is very good. No wonder it tangles at 700 km ;).

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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